1. Field of the Invention
The field of the invention is optical sensors and in particular, optical sensors suitable for use in detecting a fluctuating radiant signal in flame monitoring and similar apparatus.
2. Background Art
Optical sensors are frequently used in control systems for large boilers and furnaces to detect the presence of a flame and to provide information about the flame characteristics by measuring the radiant signal produced by the flame. Such information is used by the control system to regulate safe operation of the burner.
The constant component or average amplitude of the radiant signal, measured by the optical sensor, provides a first indication of the presence or absence of the flame. However, the average amplitude may vary considerably dependent on variables such as damper position, proximity of the flame to the sensor, the type of fuel, and the BTU content of the fuel. Further, in multiple flame systems, the radiant background can confuse systems sensitive only to average amplitude, as to the presence and state of the flame.
More sophisticated flame monitoring systems measure the amplitudes of particular frequency components of the radiant signal to provide additional information about the flame and a more accurate appraisal of flame condition. Ideally, the optical sensors used with such systems are equally sensitive to fluctuations in radiant energy regardless of the level of the radiant background, that is, the optical sensors are linear.
Optical sensors used in systems which measure the short term fluctuations in the radiant signal are typically photoresistive devices such as those using lead sulfide or other known photoresistive materials. Such photoresistive devices are relatively inexpensive and rugged and preferred over semiconductor or thermopile-type sensors which do not have sufficient sensitivity in the infrared range. Unfortunately, such photoresistive device are also non-linear in the range of illumination required for use in flame monitoring applications. Specifically, they exhibit a decreased sensitivity t fluctuations in the radiant signal as the average amplitude of the radiant signal increases.
Although methods exist for correcting for such non-linearities with compensation circuits, such methods are relatively expensive and in their complexity may reduce the reliability of the flame monitoring system. These methods also result in a reduction of signal-to-noise ratio in the ultimately produced signal.